1. Technical Field of the Invention
The present invention pertains generally to active matrix display devices and, more particularly, relates to wiring design and electrode structure on substrates of the active matrix display devices.
2. Description of the Related Art
The prior art to which the invention is directed includes a conventionally known display device structure, in which a liquid crystal is sandwiched between two substrates, and an electric field is applied to the liquid crystal through a pair of electrodes provided on the substrates to vary optical properties of the liquid crystal for providing a visual display of information.
Operation of a liquid crystal display (LCD) having this conventionally used structure is based on molecular behaviors of the liquid crystal observed when it is subjected to an electric field. When an electric field is applied between the two substrates, molecules of the liquid crystal align with a direction parallel to, or perpendicular to, the surfaces of the substrates, for instance. Such alignment of the molecules in a specific direction causes a change in the optical properties of the liquid crystal, whereby a visual display corresponding to the applied field is obtained.
In a case where the molecules of the liquid crystal align with the field direction, or become perpendicular to the surfaces of the two substrates, the liquid crystal exhibits significant anisotropic properties with respect to the transmission of light. Optical anisotropy of this type of liquid crystal can be recognized through a comparison of images seen on an LCD screen from two different viewing angles, that is, from a direction perpendicular to the screen surface and from a direction inclined at a slight angle relative to the perpendicular direction. This optical anisotropy occurs because the line of sight aligns with crystalline axes when the LCD display is seen from the perpendicular direction, and does not align when the screen is seen from an inclined viewing angle. The anisotropic properties can be easily recognized from the fact that an image displayed on a conventional LCD device becomes unclear or dim when viewed at an oblique angle, for instance.
It is commonly known that the above phenomenon places limitations in the viewing field of the LCD device so that its angle of view is smaller than that of a cathode ray tube (CRT) or an electroluminescent (EL) display device.
To overcome the aforementioned problem, Japanese Examined Patent Application Publication No. 63-21907 discloses a structure of an LCD device, in which long axes of molecules of a liquid crystal are rotated in a plane parallel to a pair of substrates to vary optical properties of the liquid crystal. This structure provides a solution to the problem related to the viewing field as the molecular axes do not become perpendicular to the substrates.
FIG. 21 shows a display element, or pixel, of an LCD device according to a conventional structure for rotating individual molecules of a liquid crystal in a plane parallel to substrates.
The LCD device comprises gate lines 11 and source lines 12 arranged to form a grid pattern as shown in FIG. 21. Each gate line 11 is a conductor line for transmitting a signal to a gate of a thin-film transistor 13 while each source line 12 is a conductor line for transmitting an image signal to a source of the thin-film transistor 13. A pixel electrode 14 connected to a drain of the thin-film transistor 13 forms a comb pattern as does another electrode 15. These electrodes 14 and 15 are arranged in such a way that individual teeth of the former lie in spaces between individual teeth of the latter. As can be seen from FIG 21, the electrode 15 branches out from a conductor line 16 which is maintained at a specified voltage.
The comblike electrodes 14 and 15 thus arranged create an electric field oriented parallel to the surfaces of the substrates, and this makes it possible to cause the individual molecules of the liquid crystal to rotate in the plane parallel to the substrates.
In the structure shown in FIG. 21, however, part of the pixel electrode 14 runs side by side with the source line 12 in an area designated by the numeral 17. Furthermore, part of the pixel electrode 14 runs side by side with the gate line 11 in an area designated by the numeral 18. This type of close parallel runs in a conductor pattern is apt to induce mutual interference due to coupling between them. More specifically, signals are disturbed between the pixel electrode 14 and source line 12, and between the pixel electrode 14 and gate line 11, resulting in degradation in image quality.
In the structure of FIG. 21, individual comblike electrodes 15 in each column of the LCD device are connected together by a conductor line 16. FIG. 22 shows an alternative conventional structure, in which a plurality of comblike electrodes 25 in each row of an LCD device are connected together by a conductor line 26 which is maintained at a specified voltage. Even when a conductor layout shown in FIG. 22 is employed, the problem of interference between electrodes remains unsolved though.
In the structure depicted in FIG. 22, gate lines 21 and source lines 22 are arranged to form a grid pattern. Each gate line 21 is a conductor line for transmitting a signal to a gate of a thin-film transistor 23 while each source line 22 is a conductor line for transmitting an image signal to a source of the thin-film transistor 23. A pixel electrode 24 is connected to a drain of the thin-film transistor 23. Each comblike electrode 25 branches out from the conductor line 26 which is held at the specified voltage. The electrodes 24 and 25 are arranged in such a way that individual teeth of the former lie in spaces between individual teeth of the latter. The comblike electrodes 24 and 25 thus arranged create an electric field oriented parallel to the surfaces of substrates of the LCD device.
The structure of FIG. 22 is still apt to cause mutual interference between the pixel electrode 24, which forms a pixel, and the source line 22 in an area designated by the numeral 27. An area designated by the numeral 28 where the pixel electrode 24 and gate line 21 run parallel to each other is also susceptible to mutual interference.